With adjuncts ( I use them all the time) There are different types. With powder, I'd prob go the last 10 min of your boil. With a Concentrate or bottle type, I go at bottling. If Chocolate like Nibs I use, I soak the Nibs overnight in Vodka to sterilize and toss it in the Primary 5-6 days after fermentation has slowed to nothing and leave it in there for about 4 days, or it starts to draw bitters. Which I collect that muslim pitch of Nibs and use it in a Rye IPA I make as a next batch and toss that used nibs at the last 15 of my boil with the used yeast to feed the new yeast and pull the bitters from the chocolate and go Hoppy.
Lastly, when using adjuncts, if you go light as you mentioned they become an off flavor instead of a distinct flavor... The best way to measure is to draw off a cup at bottling if it is a concentrate, start with a 1/4 tsp and see if you like it, if so (do the math) if not you can pitch the cup and not ruin your batch, conversely you can add more if indeed it needs more.
So I make Choc Stout like a staple here, however most are use to a milk choc instead of a dark choc (which is bitter by nature) but I add caramel and vanilla at kegging or bottling to emulate that. However, if you do too light of Caramel, it tastes like an off flavor....
